Static Grounding for U/G Fuel Tanks

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I am working on a Gas Station where we are installing new underground tanks. They are requiring a remote and separate ground rod with a #8 wire from the 4" fill (riser) pipe at the tank to this ground rod to prevent static electricity. Shouldn't this #8 ground go back to the building grounding electrode instead of going to a remote ground rod?
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
Murphy job?

They 1st started out running back to the service, but then changed to just a ground rod. I never did find out why they made that change. I assumed they did not want a fault on the service going to the tanks.

The ones I worked on we grounded the spill bucket.
 

bphgravity

Senior Member
Location
Florida
The system you reference is likely only a supplementary grounding system and is NOT required to be bonded to the service grounding electrode system.
 

haskindm

Senior Member
Location
Maryland
This is the kind of lunacy that is infecting our trade. If these are UNDERGROUND tanks, how much more GROUNDED can they possibly be? To require a ground rod for a tank that is buried in the ground makes as much sense as requiring screen doors on submarines! 250.52(A)(7) even states that an underground tank may be used as a grounding electrode! Some of the people writing these specifications need to go back and learn some of the basics!
 

haskindm

Senior Member
Location
Maryland
Bryan,
If that is the case, I can see a requirement for bonding the isolated metal components together (similar to the bonding of the metal parts of a swimming pool), but why require a ground rod? The tank and the metal components ARE IN THE GROUND! The article 100 defition of grounding is a "connection to the earth..." How much more "connected to earth" can something be that is buried?
 

bphgravity

Senior Member
Location
Florida
I totally agree. I love it when someone asks me if they need to ground an underground metal water pipe or if the underground well casing should be grounded. The look they give when I say no, the service needs to be grounded to those electrodes, there is no need to ground something already underground is even better...

I agree there are a huge number of electricians out there that have this concept backwards and upside-down.
 
My concren was that this remote ground rod could somehow be a better ground than the building ground and if someone diconnected the #8 ground wire at the tank that there could be a spark. But, I guess that would be very unlikely with the tank being IN the ground and the conduit that feeds the tank's pump is also in the ground. So, is it safe to say that it serves no real purpose and creates no additional hazards?
 

sandsnow

Senior Member
Everything we inspect at gas stations is non-metallic or PVC coated or somehow isolated from the dirt.

Corrosion was a big problem in the past. So, no metal in contact with the dirt.

edit add: I should say even the stuff we do not inspect.
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
The grounding is to dissipate static build up when fuel is dropped in the tank. They are not trying to "ground" the tank, just get rid of the static buildup. The reason there is a concern to do this is because (on the Murphy job that I've seen this specification) the tank is fiberglass. The steel risers are backfilled with washed pea gravel or small crushed stone. How good of a ground is this?

I had an electrician working under me once that wired a submerged pump and connected the black to the ground screw and the green to one leg of the pump. When the service techs went to start up the system they called me and said the submerged pump would not work. What was happening was the pump was getting 120 volts instead of 240 volts, and the steel riser that was screwed into the tank (fiberglass) was being energized with 120 volts. No breaker tripped, no explosions, no nothing. I think this indicates the backfill is not a very good ground.

The grounding of the spill containment manhole is someone being very careful trying to prevent problems.
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
dereckbc said:
Is it not a requirement for all UG gas tanks to be fiberrglass now days?

No, it's not. But, the least expensive and most easily maintained is a fiberglass tank or a steel tank that is fiberglass coated. Even a steel tank that is not fiberglass coated has a dielectric coating to isolate it from the ground (plus sacrificial anodes). UG storage tanks for fuel are isolated from ground for corrosion protection, therefore they are a horrible ground source. Plus, any opening that a steel riser may be screwed into has a dielectric bushing installed to isolate the riser from the tank.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
hardworkingstiff,
What was happening was the pump was getting 120 volts instead of 240 volts, and the steel riser that was screwed into the tank (fiberglass) was being energized with 120 volts. No breaker tripped, no explosions, no nothing. I think this indicates the backfill is not a very good ground.
Even if the metal pipe had been connected to a driven rod, it would be very unlikely to trip the breaker.
Don
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
don_resqcapt19 said:
hardworkingstiff,

Even if the metal pipe had been connected to a driven rod, it would be very unlikely to trip the breaker.
Don

Don,

I won't argue against that, but would the static charge be dissipated? I have no idea and am not necessarily agreeing with the concept, just stating what I understand.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I won't argue against that, but would the static charge be dissipated? I have no idea and am not necessarily agreeing with the concept, just stating what I understand.
The IEEE Green Book says that a path of one million ohms or less will prevent the build up of static charges.
Don
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
don_resqcapt19 said:
The IEEE Green Book says that a path of one million ohms or less will prevent the build up of static charges.
Don

Well based on this information I would think the static build up from the fuel drop would be dissipated thru the concrete bad and the grounding is not necessary.
 
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